This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present technology. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present technology. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Coal deposits may hold significant amounts of hydrocarbon gases, such as methane, ethane, and propane, generally adsorbed onto the surface of the coal. A significant amount of natural gas reserves exist as adsorbed species within coalbeds. The natural gas from coalbeds, commonly referred to as coalbed methane (CBM), currently constitutes a major source of the natural gas production in the United States. The CBM is generally produced by depressurization of coal seams. The gas content of coalbeds is dependent on the a number of factors—coal type, native pressure, and geologic history. Maximum gas adsorption increases with increasing pressure. Some coals at high pressures may have over about 300 standard cubic feet of adsorbed methane per ton of coal.
However, huge gas volumes are trapped in certain coalbeds that cannot be effectively produced with existing technology since the coalbeds do not have sufficient permeability to permit commercial production rates. The primary pathways for gas to flow through coalbeds are through natural fractures in the coal, which are called cleats. In certain coals, especially deep coals, the cleats may be essentially closed due to the overburden compression.
A common method to improve permeability of coalbeds is using cavitation (see, for example, U.S. Pat. No. 5,147,111). The process of cavitation involves a production well which is pressurized and then rapidly depressurized to cause a partial collapse of a formation, creating a hole proximate to the well. Cavitation may be performed using several cycles with flushing of the hole between each cycle to remove cave-in debris. Generally, cavitation may allow the cleat system in the coal to relax and partially expand into the void space. In this way, the permeability of the coal can be increased.
However, even using cavitation, only a small fraction of the CBM is economically recoverable. More specifically, depressurization is limited to relatively higher permeability coalbeds. This is because as pressure is decreased, the cleats may collapse and decrease the permeability of the coalbed. Loss of permeability is particularly a concern for deep coalbeds, which may have a low initial permeability. Depressurization may also result in production of low-pressure gas needing significant power for compression to permit pipelining to market.
Previous techniques have used heating of oil shale reservoirs to perform in-situ pyrolysis for enhancing the production of hydrocarbons (see, for example, U.S. Pat. Nos. 849,524; 2,634,961; 2,732,195; 2,902,270; 4,886,118; 6,745,837; 6,913,078; 7,331,385). Generally, to perform in-situ pyrolysis, a target region is heated above about 270° C.
U.S. Patent Application No. 2008/0207970 by Meurer, et al., discloses a method for produce products with improved properties by heating an organic-rich rock formation. The heating generally pyrolyzes at least a portion of the hydrocarbons in the formation (such as kerogen), converting these hydrocarbons to hydrocarbon fluids. The hydrocarbon fluids may then be produced from the formation.
U.S. Pat. No. 3,455,391 describes a process for horizontally fracturing an earth formation. The process tends to fracture the formation vertically by injecting hot fluid at high pressure until vertical fractures form and subsequently close due to thermal expansion. A fluid is then injected at a pressure sufficient to form horizontal fractures.
Further, U.S. Pat. No. 3,613,785 describes a process for horizontally fracturing a formation. Specifically, the process forms horizontal fractures in a subsurface earth formation which tends to fracture vertically at the naturally occurring formation temperature. The process includes the steps of extending at least one well borehole into the formation and generating a vertical fracture by pressurizing said borehole. A hot fluid is injected into at least one borehole to heat the formation and the injection of hot fluid is continued until thermal stressing of the formation matrix material causes the horizontal compressive stress in the formation to exceed the vertical compressive stress therein at a location selected for a second well. The borehole of the second well is extended into the formation, and the formation is hydraulically fractured through this second well borehole to form a horizontal fracture extending therefrom into the formation.
Neither of the patents described above applied the techniques disclosed to gas producing formations. Further, the use of heat to enhance hydrocarbon production when the pyrolysis of organic matter was not a key element of a production scheme, for example, heating outside of a reservoir, was not disclosed.
Other related material may be found in at least U.S. Pat. Nos. 3,095,031; 3,127,936; 4,140,179; and Swedish Patent 121,737.